The recent discovery of superconducting metal oxides such as YBa.sub.2 Cu.sub.3 O.sub.7-y (with y &lt;0.5) having critical temperatures, T.sub.c, at least 10 to 20 K above the temperature of liquid nitrogen (77 K) has created a great deal of excitement. Heretofore known superconducting materials have much lower T.sub.c 's. These superconducting metal oxides have vast potential for use in diverse applications in a large number of electrical and electronic devices which can operate at these higher temperatures. One important use of such metal oxides is in electronic components, where the oxide materials would generally be employed as a thin film, usually less than 10.mu.m thick, deposited on an appropriate substrate.
During the past 12 months, a great deal of effort has been devoted to the preparation of superconducting films of YBa.sub.2 Cu.sub.3 O.sub.7-y on various substrates. Many different techniques for depositing the film on a substrate have been explored. The film has been grown (deposited) on substrates by physical deposition techniques including electron beam evaporation, sputtering, molecular beam epitaxy, ion beam deposition, laser deposition and by chemical deposition techniques, e.g., metalorganic deposition. However, in general, the as deposited films of such materials are nonsuperconducting. They require a post growth anneal in oxygen to become superconducting. To effect the desired superconducting properties in the film material, they are usually annealed in oxygen at a temperature of 800.degree.-950.degree. C. and for a period of a few minutes to a few hours.
If the annealing conditions are excessive, however, they can have an adverse effect on the superconducting properties of the film by causing interdiffusion of elements of the film and the substrate. The interdiffusion can lead to interactions of the elements and result in films having poor or absent superconducting properties. Annealing temperatures and times therefore have to be optimized for a given substrate and thickness of the film.
Recent publications have described various physical deposition techniques and annealing conditions which are reported to have been successfully used to produce superconducting metal oxide films on special substrates such as SrTiO.sub.3, ZrO.sub.2 and MgO. In "Preparation Of Y-Ba-Cu-) Thin Films On MgO By dc Magnetron Sputtering From A Stoichiometric Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.delta. Target", Lee et al, Appl. Phys. Lett. 51 (15), 12 Oct. 1987 it was disclosed that such thin films deposited on MgO substrates, when heated above 900.degree. C. for 1 minute become superconducting at 60 K. D. K. Lathrop et al in "Production of YBa.sub.2 Cu.sub.3 O.sub.7-y Superconducting Thin Films in Situ by High-Pressure Reactive Evaporation and Rapid Thermal Annealing", Appl. Phys. Lett. 51 (19) 9 Nov. 1987 deposited such films on Al.sub.2 O.sub.3, ZrO.sub.2 and SrTiO.sub.3. They teach therein that, after deposition, such films which did not show good superconducting properties were annealed in oxygen for 1-5 minutes at 700-900.degree. C. to improve the superconducting properties.
While the above citations are directed to films deposited by physical deposition techniques, the following citation is directed to films deposited by a chemical deposition technique, namely organometallic deposition. In "Versatile New Metalorganic Process For Preparing Superconducting Thin Films," Appl. Phy. Lett. 52 (2), 11 Jan. 1988, M. E. Gross et al disclose depositing LnBa.sub.2 Cu.sub.3 O.sub.7-x, (Ln being a rare earth element) films on MgO and ZrO.sub.2 by spin coating the film from a solution. It is taught therein that heating in O.sub.2 to a maximum temperature of 800.degree.-990.degree. C. followed by annealing at 400.degree. C. produces superconducting behavior in the film at T.sub.c (onset) of 89 K. The high temperature annealing of the film is limited to a maximum of 3 minutes in order to preclude significant interaction of the film with the substrate.
In all of these citations, the typical annealing schedule consists of a relatively slow heating to a peak temperature in the range of 850.degree. to 950.degree. C., annealing in oxygen at this temperature for a time period ranging from a few minutes to an hour, followed by a slow cooling at a rate of 1 to 3 degrees per minute. Using such an annealing schedule, YBa.sub.2 Cu.sub.3 O.sub.7-y films having T.sub.c 's above 90 K and critical current densities above 10.sup.6 A/cm.sup.2 have been reported in the literature as having been made on the particular substrates mentioned above.
For most electronic uses, however, the superconducting films must be grown on substrates employed in integrated circuits, such as Si, SiO.sub.2 and Si.sub.3 N.sub.4 substrates. Unfortunately all attempts known to us by others in the past to prepare YBa.sub.2 Cu.sub.3 O.sub.7-y films by physical deposition techniques on silicon and silicon dioxide substrates have given films having poor superconducting properties, i.e., low T.sub.c 's. When we used the typical annealing schedule described above to anneal films deposited by physical deposition on Si and SiO.sub.2 as we had used for films deposited, for example, on SrTiO.sub.3, we obtained results similar to those reported in the literature, namely, the films had poor superconducting properties. Generally, such films deposited on silicon by physical deposition have a T.sub.c less than 30 K. The poor superconducting properties are attributed to the interdiffusion of one or more of the elements of the film and substrate.
In a paper presented in November of 1987 by Robert B. Laibowitz and entitled "Vapor Deposited High T.sub.c Superconducting Oxide Thin Films", it was reported that thin films of the compound YBa.sub.2 Cu.sub.3 O.sub.y were deposited on substrates such as sapphire, MgO, ZrO.sub.2, SrTiO.sub.3, Si and SiO.sub.2. Annealing was carried out in oxygen in a conventional furnace generally at temperatures around 920.degree. C. for about 4 minutes and the samples were cooled slowly to room temperature in the furnace. The films on all of the substrates of the paper listed above, other than Si and SiO.sub.2, were reported to be superconducting. Superconducting properties were not reported for films deposited on Si or SiO.sub.2, and it must be inferred that Laibowitz was not successful in making superconducting films on the silicon and silicon dioxide substrates. Similarly, in an article in the Detroit Free Press on Mar. 17, 1988, General Electric is quoted as stating that "Previous attempts to apply a superconducting film on silicon had failed because the layers mixed in the process of heating the film, known as annealing". According to that article, scientists at General Electric addressed the problem by using a buffer layer of zirconia, a heat resistant metallic oxide. It was reported that superconductive films having a T.sub.c of 87 K were obtained by using the buffer layer of zirconia between the film and the silicon substrate.
In "High T.sub.c Superconducting Films From Metallo-Organic Precursors", W. W. Davison et al, it was reported that films of YBa.sub.2 Cu.sub.3 O.sub.7-x deposited on silicon by metalorganic deposition have a T.sub.c of about 80 K. Reportedly, the film was about 0.7 .mu.m thick and was annealed in oxygen for one hour at 900.degree. C. It was also suggested in this reference that a diffusion barrier such as silver might be used between the film and the silicon. If this work is substantiated, it represents an advance in the art but provides teachings regarding only films deposited by a non-physical deposition technique, i.e., a non-vacuum-type metalorganic deposition technique.
Herrmann et al in "Preparation and Properties of High T.sub.c Superconducting Y-Ba-Cu-O Systems", Phy. Stat. Sol. (b) 142, K53 (1987) describe the effects of subjecting (Y.sub.0.6 Ba.sub.0.4).sub.2 CuO.sub.4-x materials in bulk form (compressed cylinders) to various annealing procedures. It is disclosed therein that a 2 hour high temperature anneal in N.sub.2 followed by a 30 minute high temperature anneal in O.sub.2 followed by a 2 hour lower temperature anneal in O.sub.2 gave the material a superconducting transition of 89-95 K. The reference also teaches that the cooling down of the material "from the annealing temperatures must be sufficiently slow".